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Are community assembly rules different today than in the past?

I’ve got a new paper out today, in Nature*! Working with a large group of researchers as part of the Smithsonian ETE Community Assembly and Disassembly Through Time working group, we pulled together fossil and modern datasets spanning plants and animals, from many hundreds of millions of years ago, to thousands of years ago, to a hundred years ago and today. We show that across most dataset, the non-random species pairs tend to be aggregated. That is, most taxa are randomly associated across the landscape, but some are significantly aggregated (occur at the same sites more often than expected) or significantly segregated (occur at different sites more often than expected). But starting around 6,000 years ago, something starts to change, and we see more and more segregated pairs. Today, most significantly associated species-pairs are segregated across the landscape! So after being relatively stable for 300 million years, something has changed over roughly the past 10,000 years.

In our working group meetings over the past several years, we wracked our brain trying to figure out what might cause this change. We first went to all the usual suspects that paleobiologists think are important. We know that taphonomy is important- that is, there are processes involved in forming fossil deposits that may obscure our inferences of ecological or evolutionary processes. So we asked: Was something different about the spatial resolution of fossil vs modern data? Was the difference in time- averaging across datasets responsible for the change? Were there systematic differences in the taxonomic resolution of our datasets that might be causing this apparent switch in community assembly processes? But after collating and analyzing a dataset of all these different potential confounding variables, none of those appeared to be causing the switch from aggregated to segregated pairs. We also considered whether there was something about climatic variability within the time spans of the fossil deposits that may be causing the pairs to appear aggregated. Again, no. So we still don’t know WHAT is causing the switch from aggregated to segregated pairs, but by process of elimination we’ve settled on human activity as the most likely explanation.

Human-related activity really ramps up over the past 10,000 years-increases in human population sizes, beginning of agriculture in North America, cultivation and domestication of species, changes in land use, habitat fragmentation, and increasing predation and hunting pressure (over and above the human impacts on megafaunal populations prior to 10,000 years ago). It’s likely that not one single thing was impacting ecological communities- all of the above combined to impact them. So while there is a lot of work still to be done to nail down the exact mechanisms, our data”suggest that the rules governing the assembly of communities have recently been changed by human activity”.